The evolution of radio interface standards has been strongly focused on increased data rates, and in the Third Generation Partnership Project (3GPP) Release 7 a technology known as Multiple Input Multiple Output (MIMO) was introduced. Such technology uses multiple antennas at both the transmitter and receiver to theoretically double the downlink data rate using multiple data stream transmission.
Certain User Equipment (UE) categories are able to use MIMO technology, but other legacy UE categories are not be able to use MIMO technology. A network must be able to support both MIMO enabled and non-MIMO enabled UEs. Support for legacy UEs may be provided by transmitting all system vital information and traffic channels on a single antenna. However, if there are separate Power Amplifiers (PA) for the multiple antennas, which is typically the case, the utilization of the PAs is suboptimal. This is because one antenna may be transmitting and receiving much more data than another antenna which represents an under utilisation of resources. In effect there is no power sharing between the PAs.
The problem of uneven power sharing may be alleviated by using Butler matrices at a Base Transceiver Station (BTS), also known as the NodeB. Such Butler matrices distribute the load equally over the PAs, but have the drawback of requiring more hardware and introducing a power loss. Furthermore, Butler matrices are not always useable for power balancing if the data streams from the transmit antennas are correlated, which is the case for single stream MIMO used to support legacy UEs.
Another solution is to transmit all channels not using MIMO from the antennas through the use of Space-Time Transmit Diversity (STTD) encoding, which is an open loop transmit diversity scheme standardized in 3GPP Release 99. Such STTD encoding is supported by most UEs on the market. However, even though STTD transmission alleviates the PA power balancing problem and may be beneficial for common channels, there is less benefit for dedicated channels and in particular the High-Speed Downlink Shared Channel (HS-DSCH). The use of STTD encoding may actually harm the performance in certain cases, especially on the HS-DSCH which is a shared channel and a scheduled resource.
Typically, STTD encoding is designed to combat fast fading, but in the case of a scheduled channel, such as the HS-DSCH, STTD encoding may be harmful because the gain from scheduling stems from the fast fading as shown with reference to the experimental results shown in FIG. 4. Furthermore, demodulation equalization at the UE is more difficult when using STTD because the signal and the intra-cell interference now is transmitted from two antennas instead of one, which means that there is a higher interference rate between the antennas.